The Key Concepts of Coordination Chemistry

NEET Chemistry: Coordination Compounds – Detailed Notes and Practice Questions

Chapter 9: Coordination Compounds

1. Introduction to Coordination Compounds

  • Coordination Compounds are compounds in which a central metal atom or ion is bonded to a cluster of ions or molecules (ligands) by coordinate bonds (dative bonds). They play crucial roles in biological systems (e.g., chlorophyll in plants, haemoglobin in blood, Vitamin B-12) and in industrial processes (e.g., catalysis, electroplating, analytical reagents).

2. Definitions of Important Terms

A. Coordination Entity:

  • A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.
  • Example: [Co(NH3​)6​]3+, [Ni(CO)4​], [Fe(CN)6​]4−.

B. Central Metal Atom/Ion:

  • The atom or ion (usually a transition metal) to which a fixed number of ligands are attached. It acts as a Lewis acid (electron pair acceptor).
  • Example: Co in [Co(NH3​)6​]3+, Ni in [Ni(CO)4​], Fe in [Fe(CN)6​]4−.

C. Ligands:

  • Ions or molecules that are directly bonded to the central metal atom/ion by coordinate bonds. They act as Lewis bases (electron pair donors).
  • Classification of Ligands based on Denticity (Number of donor atoms):
    1. Monodentate (Unidentate) Ligands: Possess only one donor atom.
      • Examples: Cl−,Br−,H2​O,NH3​,CO,CN−,OH−,NO2−​.
    2. Bidentate Ligands: Possess two donor atoms.
      • Examples: Ethane-1,2-diamine (en, NH2​CH2​CH2​NH2​), Oxalate ion (C2​O42−​, ox), Glycinate ion (gly−,NH2​CH2​COO−).
    3. Polydentate Ligands: Possess more than two donor atoms.
      • Example: EDTA$^{4-}$ (Ethylenediaminetetraacetate ion) is a hexadentate ligand.
  • Chelating Ligands: Bidentate or polydentate ligands that form ring-like structures with the central metal atom. The resulting complex is called a chelate. Chelates are generally more stable than non-chelate complexes (chelate effect).
  • Ambidentate Ligands: Ligands that can coordinate to the central metal atom through two different donor atoms, but only one at a time.
    • Examples: NO2−​ (can link through N or O), SCN− (can link through S or N), CN− (can link through C or N).

D. Coordination Number (CN):

  • The number of ligand donor atoms directly bonded to the central metal atom/ion in a complex.
  • Example: In [Co(NH3​)6​]3+, CN = 6. In [Ni(CO)4​], CN = 4.

E. Coordination Sphere:

  • The central metal ion and the ligands directly attached to it are collectively termed the coordination sphere. It is usually enclosed in a square bracket [ ].
  • Example: In [Co(NH3​)6​]Cl3​, [Co(NH3​)6​]3+ is the coordination sphere.

F. Counter Ions:

  • The ions present outside the coordination sphere that balance the charge of the complex ion.
  • Example: Cl− in [Co(NH3​)6​]Cl3​, K+ in K4​[Fe(CN)6​].

G. Oxidation Number of Central Metal Atom/Ion:

  • The charge that the central metal atom/ion would carry if all the ligands are removed along with the electron pairs shared with the central atom. It is represented by a Roman numeral in parentheses.
  • Example: In [Co(NH3​)6​]3+, x+6(0)=+3⇒x=+3. So, oxidation number is +3.

H. Homoleptic and Heteroleptic Complexes:

  • Homoleptic Complex: A complex in which the central metal atom is bonded to only one kind of donor group/ligands.
    • Example: [Co(NH3​)6​]3+.
  • Heteroleptic Complex: A complex in which the central metal atom is bonded to more than one kind of donor group/ligands.
    • Example: [Co(NH3​)4​Cl2​]+.

3. Werner’s Theory of Coordination Compounds

  • Proposed by Alfred Werner in 1893.
  • Postulates:
    1. Metal atoms/ions have two types of valencies:
      • Primary Valency (Ionizable Valency): Corresponds to the oxidation state of the central metal ion. It is satisfied by negative ions (counter ions) and is ionizable. Represented by dotted lines.
      • Secondary Valency (Non-ionizable Valency or Coordination Number): Corresponds to the coordination number of the central metal ion. It is satisfied by ligands (neutral molecules or ions) and is non-ionizable. Represented by solid lines.
    2. The secondary valencies are directed towards fixed positions in space around the central metal atom, defining the definite geometry of the complex.
    3. A metal can have multiple primary valencies and a fixed secondary valency (coordination number).
    4. Negative ions can satisfy both primary and secondary valencies simultaneously (e.g., Cl− can be a counter ion and a ligand).
  • Significance: Explained the nature of bonding in coordination compounds and predicted their geometries.

4. IUPAC Nomenclature of Mononuclear Coordination Compounds

Rules:

  1. Order of Naming Ions: The cation is named first, followed by the anion.
  2. Naming of Coordination Sphere:
    • Ligands are named first in alphabetical order, followed by the central metal atom/ion.
    • Prefixes di-, tri-, tetra-, etc., are used for simple ligands. If ligands already have prefixes in their names (e.g., ethylenediamine) or are complex, then bis- (2), tris- (3), tetrakis- (4), etc., are used, and the ligand name is put in parentheses.
    • Anionic Ligands: End in ‘-o’ (e.g., Cl− (chloro), SO42−​ (sulphato), OH− (hydroxo), CN− (cyano)).
    • Neutral Ligands: Named as such (e.g., NH3​ (ammine), H2​O (aqua), CO (carbonyl), NO (nitrosyl), en (ethane-1,2-diamine)).
    • Cationic Ligands: End in ‘-ium’ (rare).
  3. Naming of Central Metal Ion:
    • If the coordination sphere is a cationic or neutral complex, the metal name is written as such (e.g., cobalt, nickel, platinum).
    • If the coordination sphere is an anionic complex, the metal name ends with ‘-ate’ (e.g., cobaltate, nickelate, ferrate (from ferrum), cuprate (from cuprum), argentate (from argentum), aurate (from aurum), plumbate (from plumbum)).
  4. Oxidation State: The oxidation state of the central metal atom is indicated by a Roman numeral in parentheses after the metal name.
  5. Ambidentate Ligands: The point of attachment is specified by writing the symbol of the donor atom after the ligand name (e.g., nitrito-N- or nitrito-O-).

Examples:

  • [Co(NH3​)6​]Cl3​: Hexaamminecobalt(III) chloride
  • K4​[Fe(CN)6​]: Potassium hexacyanoferrate(II)
  • [Co(NH3​)4​Cl2​]Cl: Tetraamminedichlorocobalt(III) chloride
  • [Ni(CO)4​]: Tetracarbonylnickel(0)

5. Isomerism in Coordination Compounds

Isomers are compounds that have the same chemical formula but different arrangements of atoms.

A. Structural Isomerism (Different connectivity):

  1. Ionization Isomerism: Occurs when the counter ion in a complex is a potential ligand and can swap positions with a ligand inside the coordination sphere.
    • Example: [Co(NH3​)5​Br]SO4​ (pentaamminebromocobalt(III) sulphate) and [Co(NH3​)5​SO4​]Br (pentaamminesulphatocobalt(III) bromide).
    • They give different ions in solution.
  2. Hydrate Isomerism (Solvate Isomerism): Special case of ionization isomerism where water is involved as a solvent. Water molecule can be a ligand or a solvent molecule outside the coordination sphere.
    • Example: [Cr(H2​O)6​]Cl3​ (violet) and [Cr(H2​O)5​Cl]Cl2​⋅H2​O (grey-green).
  3. Linkage Isomerism: Occurs in complexes containing ambidentate ligands (ligands that can coordinate through two different donor atoms).
    • Example: [Co(NH3​)5​NO2​]Cl2​ (pentaamminenitrito-N-cobalt(III) chloride, yellow) and [Co(NH3​)5​ONO]Cl2​ (pentaamminenitrito-O-cobalt(III) chloride, red).
  4. Coordination Isomerism: Occurs in compounds where both cation and anion are complex entities, and the ligands are exchanged between the cation and anion.
    • Example: [Co(NH3​)6​][Cr(CN)6​] and [Cr(NH3​)6​][Co(CN)6​].

B. Stereoisomerism (Same connectivity, different spatial arrangement):

  1. Geometrical Isomerism (Cis-Trans Isomerism): Arises due to different possible geometric arrangements of ligands around the central metal atom.
    • Not possible for coordination number 2 and 3.
    • Square Planar Complexes ([Ma2​b2​], [Ma2​bc], [Mabcd]): Exhibit cis-trans isomerism.
      • Cis-isomer: Identical ligands are adjacent.
      • Trans-isomer: Identical ligands are opposite.
    • Octahedral Complexes ([Ma4​b2​], [Ma3​b3​], [Ma2​b2​c2​], [M(AA)2​b2​] where AA is bidentate):
      • [Ma4​b2​]: Cis-trans isomers.
      • [Ma3​b3​] (Fac-Mer Isomerism):
        • Fac (facial) isomer: Three identical ligands occupy adjacent positions at the corners of an octahedral face.
        • Mer (meridional) isomer: Three identical ligands occupy positions around the meridian of the octahedron.
    • Not generally observed for tetrahedral geometry.
  2. Optical Isomerism (Enantiomerism): Arises when a complex is non-superimposable on its mirror image (chiral). Such isomers rotate plane-polarized light in opposite directions.
    • Common in octahedral complexes, especially those with bidentate ligands.
    • Examples: [Co(en)3​]3+, [Co(en)2​Cl2​]+.
    • Square planar complexes usually do not show optical isomerism (due to presence of plane of symmetry).

6. Bonding in Coordination Compounds

A. Werner’s Theory: (Already discussed above)

B. Valence Bond Theory (VBT):

  • Proposed by Pauling. Explains bonding, geometry, and magnetic properties.
  • Postulates:
    1. The central metal atom/ion provides vacant s, p, and d atomic orbitals for hybridization.
    2. Hybridized orbitals overlap with filled ligand orbitals to form coordinate covalent bonds.
    3. The geometry of the complex is determined by the type of hybridization (e.g., sp$^3$ for tetrahedral, dsp$^2$ for square planar, sp$^3d^2$ or d$^2sp^3$ for octahedral).
    4. Magnetic Properties:
      • If the complex has unpaired electrons, it is paramagnetic.
      • If all electrons are paired, it is diamagnetic.
    5. Inner Orbital (Low Spin) Complex: When the (n−1)d orbitals are used for hybridization. Strong field ligands cause pairing of electrons.
    6. Outer Orbital (High Spin) Complex: When the nd orbitals are used for hybridization. Weak field ligands do not cause pairing of electrons.
  • Limitations of VBT:
    1. Does not explain the colour of coordination compounds.
    2. Does not provide a quantitative explanation for magnetic properties.
    3. Cannot explain why certain ligands are strong field and others are weak field.
    4. Does not explain the relative stability of complexes.

C. Crystal Field Theory (CFT):

  • Proposed by Bethe and Van Vleck. Focuses on the interaction between the metal ion and ligands as purely electrostatic (ionic) interactions.
  • Postulates:
    1. Ligands are treated as point charges (anions) or dipoles (neutral molecules).
    2. The interaction between the central metal ion and ligands is purely electrostatic.
    3. The d-orbitals on the free metal ion are degenerate (have the same energy).
    4. When ligands approach the metal ion, the d-orbitals lose their degeneracy due to the electrostatic field of the ligands. This splitting of degenerate d-orbitals is called crystal field splitting.
  • Crystal Field Splitting in Octahedral Complexes:
    • The five d-orbitals split into two sets:
      • eg​ set: Consists of dx2−y2​ and dz2​ orbitals, which point directly along the axes towards the approaching ligands. These experience greater repulsion and are raised in energy.
      • t2g​ set: Consists of dxy​,dyz​,dzx​ orbitals, which lie between the axes. These experience less repulsion and are lowered in energy.
    • The energy difference between the t2g​ and eg​ sets is called the crystal field splitting energy (Δo​ for octahedral).
    • Factors affecting Δo​:
      1. Nature of Ligand (Spectrochemical Series): The relative strength of ligands in causing crystal field splitting. Strong field ligands cause large splitting (large Δo​), weak field ligands cause small splitting (small Δo​). Spectrochemical series: CO,CN−>en>NH3​>H2​O>ox2−>OH−>F−>Cl−>Br−>I− (Strong field ligands on left, weak field ligands on right)
      2. Oxidation State of Metal Ion: Higher the charge on the metal ion, greater the splitting.
      3. Nature of Metal Ion: For a given ligand, Δo​ increases down a group (3d < 4d < 5d).
  • High Spin vs. Low Spin Complexes (based on Δo​ vs Pairing Energy, P):
    • When electrons fill the d-orbitals, they either pair up in lower energy t2g​ orbitals (if Δo​>P, strong field ligands) or occupy eg​ orbitals first (if Δo​<P, weak field ligands).
    • Strong field ligands: Cause low spin (spin paired) complexes. Electrons prefer to pair up in t2g​ orbitals.
    • Weak field ligands: Cause high spin (spin free) complexes. Electrons prefer to occupy eg​ orbitals unpaired before pairing occurs in t2g​.
  • Crystal Field Splitting in Tetrahedral Complexes:
    • Ligands approach between the axes.
    • The splitting is inverted compared to octahedral: e orbitals are lowered in energy, and t2​ orbitals are raised in energy.
    • Δt​<Δo​ (approximately Δt​=94​Δo​).
    • Always forms high spin complexes (as Δt​ is usually small, P>Δt​).
  • Colour of Coordination Compounds (explained by CFT):
    • The colour arises from d-d transitions. When white light falls on a complex, electrons in the lower energy d-orbitals absorb light of a specific wavelength and jump to higher energy d-orbitals. The colour observed is the complementary colour of the light absorbed.
  • Limitations of CFT:
    1. Does not consider the covalent character of bonding between metal and ligands.
    2. Does not explain the bonding in carbonyls and other complexes where the metal is in a low oxidation state.
    3. Treats ligands as point charges, but neutral ligands like CO and NH3​ still cause splitting.

7. Stability of Coordination Compounds

  • Thermodynamic Stability: Refers to the extent to which a complex will form or dissociate at equilibrium. It is expressed by the stability constant (K) or formation constant (β).
    • M+nL⇌MLn​ ; K=[MLn​]/([M][L]n)
    • Higher the K value, more stable the complex.
  • Factors Affecting Stability:
    1. Nature of Central Metal Ion:
      • Higher the charge on the metal ion, greater the stability.
      • Smaller the size of the metal ion (for same charge), greater the stability.
      • For a given ligand and oxidation state, stability generally increases down a group (3d < 4d < 5d).
    2. Nature of Ligand:
      • Basicity of Ligand: Stronger bases (ligands with higher electron-donating ability) form more stable complexes.
      • Chelate Effect: Chelating ligands form more stable complexes than non-chelating (monodentate) ligands. This is due to an increase in entropy.
      • Number of Rings: More the number of rings formed by chelating ligands, greater the stability.

8. Importance and Applications of Coordination Compounds

  1. Analytical Chemistry:
    • Quantitative analysis (e.g., EDTA for hardness of water, estimation of Ni2+).
    • Qualitative analysis (e.g., Prussian blue test for Fe3+, detection of Ni2+ using dimethylglyoxime).
  2. Biological Systems:
    • Chlorophyll: Magnesium complex, essential for photosynthesis.
    • Haemoglobin: Iron complex, oxygen transport in blood.
    • Vitamin B$_{12}$: Cobalt complex, involved in red blood cell formation.
    • Enzymes (e.g., Carboxypeptidase A contains Zn).
  3. Metallurgy: Extraction of metals (e.g., Ag and Au by cyanide process).
  4. Industrial Catalysis: Wilkinson’s catalyst ([(Ph3​P)3​RhCl]) for hydrogenation of alkenes.
  5. Electroplating: Deposition of metals (e.g., Ag, Au, Cr) using their complex ions.
  6. Medicine:
    • Cisplatin ([Pt(NH3​)2​Cl2​]): Antitumour agent in cancer therapy.
    • Chelating agents are used to remove toxic metal ions from the body (e.g., EDTA for lead poisoning).
  7. Photography: In fixing process, unreacted AgBr is removed as a soluble complex Na3​[Ag(S2​O3​)2​].

NEET Chemistry: Coordination Compounds – Practice Questions

I. Multiple Choice Questions (MCQs)

1. Question: The coordination number and oxidation state of Co in [Co(en)2​Cl2​]Cl are respectively: a) 4, +2 b) 6, +3 c) 4, +3 d) 6, +2

2. Question: Which of the following is an ambidentate ligand? a) H2​O b) NH3​ c) NO2−​ d) ox2−

3. Question: The IUPAC name of K3​[Fe(C2​O4​)3​] is: a) Potassium trioxalatoferrate(III) b) Tripotassium trioxalatoiron(III) c) Potassium trioxalatoferrate(II) d) Potassium iron(III) trioxalate

4. Question: Which of the following exhibits linkage isomerism? a) [Co(NH3​)5​Br]SO4​ b) [Cr(H2​O)6​]Cl3​ c) [Co(NH3​)5​NO2​]Cl2​ d) [Co(NH3​)6​][Cr(CN)6​]

5. Question: The shape of [NiCl4​]2− is tetrahedral, while [Ni(CN)4​]2− is square planar. This is because: a) Cl− is a strong field ligand, and CN− is a weak field ligand. b) CN− is a strong field ligand, and Cl− is a weak field ligand. c) Both are strong field ligands. d) Both are weak field ligands.

6. Question: The crystal field splitting energy (Δo​) for octahedral complexes depends on: a) Nature of ligand b) Oxidation state of metal ion c) Nature of metal ion d) All of the above

7. Question: Which of the following is a homoleptic complex? a) [Co(NH3​)4​Cl2​]+ b) [Cr(en)2​Cl2​]+ c) [Ni(CO)4​] d) [Pt(NH3​)Cl3​]−

8. Question: The complex [Co(NH3​)4​Cl2​]Br shows: a) Ionization isomerism b) Linkage isomerism c) Geometrical isomerism d) Both (a) and (c)

9. Question: Which of the following complex ions is expected to be diamagnetic? a) [Ti(H2​O)6​]3+ (Atomic No. of Ti = 22) b) [Ni(CN)4​]2− (Atomic No. of Ni = 28) c) [Fe(H2​O)6​]2+ (Atomic No. of Fe = 26) d) [CoF6​]3− (Atomic No. of Co = 27)

10. Question: Which of the following does NOT show optical isomerism? a) [Cr(en)3​]3+ b) [Co(en)2​Cl2​]+ (cis-isomer) c) [Pt(en)Cl2​] (square planar) d) [Co(gly)3​]

11. Question: According to Werner’s theory, primary valency corresponds to: a) Coordination number b) Oxidation state c) Both oxidation state and coordination number d) Number of ligands

12. Question: The colour of coordination compounds is mainly attributed to: a) Presence of unpaired electrons b) Intermolecular hydrogen bonding c) d-d transitions d) Charge transfer spectra

13. Question: Which of the following ligands causes maximum crystal field splitting? a) Cl− b) H2​O c) NH3​ d) CN−

14. Question: Which of the following is used in cancer therapy? a) Vitamin B$_{12}$ b) Cisplatin c) Chlorophyll d) Haemoglobin

15. Question: The chelate effect refers to: a) Formation of complexes with more than one type of ligand. b) Increased stability of complexes with chelating ligands. c) Magnetic properties of complexes. d) Colour of coordination compounds.

II. Assertion-Reason Type Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct option. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is NOT the correct explanation of A. c) A is true but R is false. d) A is false but R is true.

16. Assertion (A): [Ni(CN)4​]2− is diamagnetic, while [NiCl4​]2− is paramagnetic. Reason (R): CN− is a strong field ligand, causing pairing of electrons, while Cl− is a weak field ligand.

17. Assertion (A): Chelating ligands make complexes more stable. Reason (R): Chelating ligands increase the entropy of the system.

18. Assertion (A): Optical isomerism is not shown by square planar complexes. Reason (R): Square planar complexes have a plane of symmetry.

19. Assertion (A): Transition metals form a large number of coordination compounds. Reason (R): Transition metals have small size, high charge, and vacant d-orbitals.

20. Assertion (A): K4​[Fe(CN)6​] is called potassium hexacyanoferrate(II). Reason (R): The central metal ion in an anionic complex is given the suffix ‘-ate’.

III. Short Answer / Conceptual Questions

21. Question: Define ligands and classify them based on denticity with one example for each type.

22. Question: What are ambidentate ligands? Give two examples and indicate the donor atoms.

23. Question: State any two postulates of Werner’s theory of coordination compounds.

24. Question: Write the IUPAC names for the following coordination compounds: a) [Cr(NH3​)3​Cl3​] b) Na2​[Ni(CN)4​]

25. Question: Differentiate between inner orbital complex and outer orbital complex based on VBT.

26. Question: Explain crystal field splitting in an octahedral complex. Draw the d-orbital splitting diagram.

27. Question: How does Crystal Field Theory explain the colour of coordination compounds?

28. Question: Draw the geometrical isomers (cis and trans) for the complex ion [Co(en)2​Cl2​]+.

29. Question: Explain why [CoF6​]3− is paramagnetic while [Co(NH3​)6​]3+ is diamagnetic. (Atomic No. of Co = 27)

30. Question: List any three important applications of coordination compounds in everyday life or industry.

Answers and Explanations

I. Multiple Choice Questions (MCQs) – Answers

1. Answer: b) 6, +3 Explanation:

  • Coordination Number: ‘en’ (ethane-1,2-diamine) is a bidentate ligand, so two ‘en’ ligands contribute 2×2=4 donor atoms. There are two Cl− ligands, contributing 2×1=2 donor atoms. Total coordination number = 4+2=6.
  • Oxidation State: Let Co be x. (en) is neutral (0 charge). Cl− has -1 charge. The complex has +1 charge (due to one Cl− counter ion). x+2(0)+2(−1)=+1 x−2=+1⇒x=+3.

2. Answer: c) NO2−​ Explanation: An ambidentate ligand can coordinate through two different donor atoms. NO2−​ can coordinate through nitrogen (nitro-N or nitro) or through oxygen (nitrito-O or nitrito). H2​O and NH3​ are monodentate, and ox2− is bidentate.

3. Answer: a) Potassium trioxalatoferrate(III) Explanation:

  • Cation first: Potassium.
  • Coordination sphere is anionic.
  • Ligand: Oxalate (C2​O42−​), it’s a bidentate ligand, three are present, so ‘trioxalato’.
  • Metal: Iron, in anionic complex ‘ferrate’.
  • Oxidation state: Let Fe be x. 3(+1)+x+3(−2)=0⇒3+x−6=0⇒x=+3. So, Ferrate(III). Combining: Potassium trioxalatoferrate(III).

4. Question: c) [Co(NH3​)5​NO2​]Cl2​ Explanation: Linkage isomerism arises in complexes containing ambidentate ligands. NO2−​ is an ambidentate ligand (can bind via N or O). a) Ionization isomerism. b) Hydrate isomerism. d) Coordination isomerism.

5. Answer: b) CN− is a strong field ligand, and Cl− is a weak field ligand. Explanation: For Ni2+ (d$^8$ configuration):

  • CN− is a strong field ligand. In the presence of a strong field ligand, electrons pair up, leading to dsp$^2$ hybridization and a square planar geometry for d8 complexes, which is diamagnetic.
  • Cl− is a weak field ligand. In the presence of a weak field ligand, electrons do not pair up, leading to sp$^3$ hybridization and a tetrahedral geometry for d8 complexes, which is paramagnetic.

6. Answer: d) All of the above Explanation: Crystal field splitting energy (Δo​) is influenced by the nature of the ligand (stronger ligands cause greater splitting), the oxidation state of the metal ion (higher charge causes greater splitting), and the nature of the metal ion (splitting increases down a group for transition metals).

7. Answer: c) [Ni(CO)4​] Explanation: A homoleptic complex has only one kind of ligand. In [Ni(CO)4​], only carbonyl (CO) ligands are present. The other options have multiple types of ligands: [Co(NH3​)4​Cl2​]+, [Cr(en)2​Cl2​]+, [Pt(NH3​)Cl3​]−.

8. Question: d) Both (a) and (c) Explanation: The complex is [Co(NH3​)4​Cl2​]Br.

  • Ionization isomerism: It can show ionization isomerism with [Co(NH3​)4​ClBr]Cl (where Cl− inside and Br− outside can swap positions).
  • Geometrical isomerism: The complex ion [Co(NH3​)4​Cl2​]+ can exist as cis and trans isomers (due to the arrangement of two Cl ligands). It does not show linkage isomerism (no ambidentate ligand) or optical isomerism (cis form is superimposable, trans form has a plane of symmetry).

9. Answer: b) [Ni(CN)4​]2− (Atomic No. of Ni = 28) Explanation: A diamagnetic complex has no unpaired electrons.

  • a) Ti3+: Z=22, [Ar]3d1. (1 unpaired e-). Paramagnetic.
  • b) Ni2+: Z=28, [Ar]3d8. In [Ni(CN)4​]2−, CN− is a strong field ligand. It causes pairing of electrons in 3d8 system to form a square planar complex (dsp$^2$ hybridization). All 8 electrons are paired. Diamagnetic.
  • c) Fe2+: Z=26, [Ar]3d6. In [Fe(H2​O)6​]2+, H2​O is a weak field ligand, resulting in high spin (4 unpaired e-). Paramagnetic.
  • d) Co3+: Z=27, [Ar]3d6. In [CoF6​]3−, F− is a weak field ligand, resulting in high spin (4 unpaired e-). Paramagnetic.

10. Answer: c) [Pt(en)Cl2​] (square planar) Explanation: Square planar complexes generally do not show optical isomerism because they possess a plane of symmetry, making them superimposable on their mirror images. All other options are octahedral complexes that can exist as chiral forms and show optical isomerism.

11. Answer: b) Oxidation state Explanation: According to Werner’s theory, primary valency corresponds to the oxidation state of the central metal ion and is ionizable. Secondary valency corresponds to the coordination number and is non-ionizable.

12. Answer: c) d-d transitions Explanation: The characteristic colours of transition metal coordination compounds are primarily due to d-d transitions. In the presence of ligands, the degenerate d-orbitals split into different energy levels. Electrons absorb specific wavelengths of visible light to jump from lower to higher energy d-orbitals, and the complementary colour is observed.

13. Answer: d) CN− Explanation: Based on the spectrochemical series, CN− (cyanide) is a very strong field ligand, causing the largest crystal field splitting among the given options. The spectrochemical series is CO,CN−>en>NH3​>H2​O>ox2−>OH−>F−>Cl−>Br−>I−.

14. Answer: b) Cisplatin Explanation: Cisplatin ([Pt(NH3​)2​Cl2​]) is a well-known coordination compound used as an anticancer drug, particularly in the treatment of various solid tumours.

15. Answer: b) Increased stability of complexes with chelating ligands. Explanation: The chelate effect is the enhanced stability of a complex containing a chelating ligand (a bidentate or polydentate ligand that forms a ring structure with the metal ion) compared to a similar complex with monodentate ligands. This increased stability is primarily attributed to a more favourable entropy change.

II. Assertion-Reason Type Questions – Answers

16. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Assertion (A) is true. For Ni2+ (d$^8$), CN− is a strong field ligand according to the spectrochemical series, causing pairing of electrons in the d-orbitals and resulting in a diamagnetic square planar complex. Cl− is a weak field ligand, leading to unpaired electrons and a paramagnetic tetrahedral complex. Reason (R) accurately explains this difference in magnetic properties and geometry.

17. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Chelating ligands form more stable complexes (chelate effect). This increased stability is largely due to the positive entropy change (increase in disorder) that occurs when a chelating ligand replaces several monodentate ligands. For example, one bidentate ligand replaces two monodentate ligands, resulting in an increase in the number of free particles in solution, leading to a favourable entropy change.

18. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Square planar complexes typically do not show optical isomerism. This is because square planar geometry inherently possesses a plane of symmetry (the plane containing the metal ion and all four ligands), which means the molecule is superimposable on its mirror image and thus not chiral.

19. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Transition metals readily form a large number of coordination compounds. This is due to their characteristics: small size of their ions, high positive charge (allowing strong attraction for ligands), and the availability of vacant d-orbitals of appropriate energy to accept lone pairs of electrons from ligands to form coordinate bonds.

20. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: The IUPAC naming convention for anionic coordination complexes dictates that the name of the central metal ion ends with the suffix ‘-ate’. For iron, the Latin root ‘ferr-‘ is used, so it becomes ‘ferrate’. Hence, the name potassium hexacyanoferrate(II) is correct, and the Reason explains the rule.

III. Short Answer / Conceptual Questions – Answers

21. Answer: Ligands: Ligands are ions or molecules that are directly bonded to the central metal atom or ion in a coordination entity. They act as Lewis bases, donating a lone pair of electrons to the central metal atom/ion to form coordinate bonds. Classification based on Denticity:

  1. Monodentate (Unidentate) Ligands: Possess only one donor atom through which they can coordinate to the central metal atom/ion.
    • Example: Cl− (chloride), NH3​ (ammine), H2​O (aqua), CO (carbonyl).
  2. Bidentate Ligands: Possess two donor atoms that can simultaneously coordinate to the central metal atom/ion.
    • Example: Ethane-1,2-diamine (en, NH2​CH2​CH2​NH2​, two N donor atoms), Oxalate ion (ox2−, C2​O42−​, two O donor atoms).
  3. Polydentate Ligands: Possess more than two donor atoms that can simultaneously coordinate to the central metal atom/ion.
    • Example: Ethylenediaminetetraacetate ion (EDTA$^{4-}$), which is a hexadentate ligand (four O and two N donor atoms).

22. Answer: Ambidentate Ligands: These are ligands that have two different donor atoms but can coordinate to the central metal atom/ion through only one of the donor atoms at a time. The choice of donor atom depends on the nature of the metal ion and reaction conditions. Two Examples and their donor atoms:

  1. Nitrito (NO2−​): Can bind through the nitrogen atom (as nitro, -NO$_2$) or through one of the oxygen atoms (as nitrito, -ONO).
    • Donor atoms: Nitrogen (N) or Oxygen (O).
  2. Thiocyanato (SCN−): Can bind through the sulfur atom (as thiocyanato, -SCN) or through the nitrogen atom (as isothiocyanato, -NCS).
    • Donor atoms: Sulfur (S) or Nitrogen (N).

23. Answer: Werner’s theory of coordination compounds, proposed by Alfred Werner in 1893, laid the foundation for understanding the bonding and structure of these complexes. Two key postulates are:

  1. Dual Valencies: Metal atoms/ions possess two types of valencies:
    • Primary Valency: Corresponds to the oxidation state of the central metal ion. It is satisfied by negatively charged ions (counter ions) and is ionizable.
    • Secondary Valency: Corresponds to the coordination number of the central metal ion. It is satisfied by ligands (neutral molecules or ions) and is non-ionizable. These secondary valencies are fixed for a particular metal ion.
  2. Spatial Arrangement (Geometry): The secondary valencies are directed towards fixed positions in space around the central metal atom. This specific spatial arrangement defines the definite geometry of the coordination complex (e.g., octahedral, tetrahedral, square planar).

24. Answer: a) [Cr(NH3​)3​Cl3​]: * Ligands: Ammine (NH3​, neutral) and Chloro (Cl−, anionic). Three of each. Alphabetical order: ammine before chloro. * Metal: Chromium (neutral complex). * Oxidation state of Cr: x+3(0)+3(−1)=0⇒x=+3. * IUPAC Name: Triamminetrichlorochromium(III)

b) Na2​[Ni(CN)4​]: * Cation first: Sodium. (Since there are 2 sodium ions, indicate ‘disodium’ but in common practice, simply ‘sodium’ for metal counter-ions). * Coordination sphere is anionic. * Ligand: Cyano (CN−, anionic). Four are present, so ‘tetracyano’. * Metal: Nickel, in anionic complex ‘nickelate’. * Oxidation state of Ni: 2(+1)+x+4(−1)=0⇒2+x−4=0⇒x=+2. * IUPAC Name: Sodium tetracyanonickelate(II)

25. Answer: The distinction between inner orbital and outer orbital complexes is made in Valence Bond Theory (VBT) based on which d-orbitals are used for hybridization: | Feature | Inner Orbital Complex (Low Spin Complex) | Outer Orbital Complex (High Spin Complex) | | :—————— | :———————————————– | :———————————————— | | d-orbitals used | Uses (n−1)d orbitals for hybridization. | Uses nd orbitals for hybridization. | | Hybridization | Typically d$^2sp^3$ (for octahedral) | Typically sp$^3d^2$ (for octahedral) | | Ligand Strength | Formed in the presence of strong field ligands. | Formed in the presence of weak field ligands. | | Electron Pairing | Strong field ligands cause pairing of electrons in (n−1)d orbitals (if required). | Weak field ligands do not cause pairing of electrons; electrons remain unpaired if possible. | | Spin State | Usually has fewer unpaired electrons (low spin). | Usually has more unpaired electrons (high spin). |

26. Answer: Crystal Field Splitting in an Octahedral Complex: In a free metal ion, the five d-orbitals (dxy​,dyz​,dzx​,dx2−y2​,dz2​) are degenerate (have the same energy). When ligands approach a central metal ion to form an octahedral complex, they approach along the x, y, and z axes. The d-orbitals whose lobes point directly along these axes (dx2−y2​ and dz2​) experience greater electrostatic repulsion from the negatively charged ligands (or negative ends of dipoles). Consequently, their energy increases. The d-orbitals whose lobes lie between the axes (dxy​,dyz​,dzx​) experience less repulsion from the ligands, and their energy decreases. This results in the splitting of the five degenerate d-orbitals into two sets:

  • eg​ set: Consists of the higher energy dx2−y2​ and dz2​ orbitals.
  • t2g​ set: Consists of the lower energy dxy​,dyz​,dzx​ orbitals. The energy difference between the eg​ and t2g​ sets is called the crystal field splitting energy, denoted as Δo​ (or 10Dq).

d-orbital Splitting Diagram for Octahedral Complex:

       E
       ^
       |
       |     --- $e_g$ ($d_{x^2-y^2}, d_{z^2}$)
       |    /  ^
       |   /   |   (Energy = +0.6 $\Delta_o$)
       |  /    |
       |  ----- (Barycentre - Average Energy of d-orbitals)
       |  \    |
       |   \   |   (Energy = -0.4 $\Delta_o$)
       |    \  v
       v     --- $t_{2g}$ ($d_{xy}, d_{yz}, d_{zx}$)

27. Answer: Crystal Field Theory (CFT) successfully explains the colour of coordination compounds. The explanation is based on the phenomenon of d-d transitions.

  1. Crystal Field Splitting: In a complex, the degeneracy of the d-orbitals of the central metal ion is removed due to the electrostatic field created by the ligands. This results in the splitting of d-orbitals into different energy levels (e.g., t2g​ and eg​ in octahedral complexes).
  2. Absorption of Light: When white light (which contains all colours of the visible spectrum) falls on the coordination compound, the electrons present in the lower energy d-orbitals (t2g​ in octahedral) absorb a specific wavelength (or colour) of visible light. This absorbed energy corresponds to the crystal field splitting energy (Δo​).
  3. Excitation (d-d Transition): By absorbing this energy, the electrons get excited (jump) from the lower energy d-orbitals to the higher energy d-orbitals (eg​ in octahedral). This process is known as a d-d transition.
  4. Emission of Complementary Colour: The complex then emits the remaining (unabsorbed) light. The colour that we perceive is the complementary colour of the light that was absorbed. For example, if a complex absorbs red light, it will appear green.

28. Answer: The complex ion is [Co(en)2​Cl2​]+. ‘en’ (ethane-1,2-diamine) is a bidentate ligand. The complex is octahedral. It can exist in two geometrical isomeric forms: cis and trans. a) Trans-isomer: The two identical ligands (Cl−) are opposite to each other (at 180∘). This isomer has a plane of symmetry.

     Cl
     |
    en - Co - en
     |
    Cl

(Simplified representation: en groups are in the plane, Cl are axial and opposite) More accurately:

      Cl
      |
    N - Co - N
   /         \
  N           N
   \         /
    N - Co - N
      |
      Cl

(Showing bonds, where ‘en’ bridges the N-N links. The two Cl’s are opposite.)

b) Cis-isomer: The two identical ligands (Cl−) are adjacent to each other (at 90∘). This isomer is chiral and does not have a plane of symmetry, hence it can show optical isomerism.

      Cl
     /
    en - Co - en
   /
  Cl

(Simplified representation: both Cl’s are on one side, en groups fill the rest) More accurately:

      Cl
     /
    N - Co - N
   / /
  N N
  \ /
   Cl

(Showing bonds, where ‘en’ bridges the N-N links. The two Cl’s are adjacent.)

29. Answer: To explain the magnetic properties, we need to determine the number of unpaired electrons in the central metal ion (Cobalt, Co, Atomic No. 27).

  • Electronic Configuration of Co: [Ar]3d74s2.
  • Oxidation state of Co in both complexes: Let Co be x.
    • For [CoF6​]3−: x+6(−1)=−3⇒x−6=−3⇒x=+3.
    • For [Co(NH3​)6​]3+: x+6(0)=+3⇒x=+3. So, in both complexes, Cobalt is in the Co3+ oxidation state.
  • Electronic Configuration of Co3+ ion: Remove two 4s electrons and one 3d electron from Co. So, Co3+ is [Ar]3d6.

Now, we consider the effect of ligands (based on Crystal Field Theory):

  1. [CoF6​]3−:
    • Ligand: F− (Fluoride ion). According to the spectrochemical series, F− is a weak field ligand.
    • In the presence of a weak field ligand, the crystal field splitting energy (Δo​) is small, and the electrons prefer to occupy higher energy orbitals before pairing up (i.e., Pairing Energy P > Δo​).
    • For d6 configuration in an octahedral complex with a weak field ligand (high spin complex): The electrons will fill t2g​ first (3 electrons), then eg​ (2 electrons), then pair up in t2g​ (1 electron). This results in: t2g4​eg2​.
    • Number of unpaired electrons = 4.
    • Therefore, [CoF6​]3− is paramagnetic.
  2. [Co(NH3​)6​]3+:
    • Ligand: NH3​ (Ammine). According to the spectrochemical series, NH3​ is a strong field ligand.
    • In the presence of a strong field ligand, the crystal field splitting energy (Δo​) is large, and electrons prefer to pair up in lower energy orbitals before occupying higher energy ones (i.e., P < Δo​).
    • For d6 configuration in an octahedral complex with a strong field ligand (low spin complex): All 6 electrons will pair up in the lower energy t2g​ orbitals. t2g6​eg0​.
    • Number of unpaired electrons = 0.
    • Therefore, [Co(NH3​)6​]3+ is diamagnetic.

30. Question: Three important applications of coordination compounds in everyday life or industry are:

  1. Biological Systems: Coordination compounds are essential components of various biological systems. For instance:
    • Chlorophyll, the green pigment in plants responsible for photosynthesis, is a coordination complex of Magnesium (Mg).
    • Haemoglobin, the oxygen-carrying pigment in red blood cells, is a coordination complex of Iron (Fe).
    • Vitamin B$_{12}$ (cyanocobalamin) is a coordination complex of Cobalt (Co), crucial for blood formation and neurological function.
  2. Medicine and Analytical Chemistry:
    • Cisplatin ([Pt(NH3​)2​Cl2​]) is a prominent example of a coordination compound used as an effective antitumour agent in cancer chemotherapy.
    • EDTA (Ethylenediaminetetraacetic acid), a chelating ligand, is used in medicine for treating metal poisoning (chelation therapy), for example, to remove toxic lead (Pb) from the body. In analytical chemistry, EDTA is widely used for the volumetric estimation of metal ions (e.g., in determining the hardness of water).
  3. Industrial Catalysis and Metallurgy:
    • Many coordination compounds act as catalysts in various industrial processes. For example, Wilkinson’s catalyst ([(Ph3​P)3​RhCl]), a rhodium complex, is used for the hydrogenation of alkenes.
    • In metallurgy, coordination compounds are involved in the extraction and purification of metals. For example, silver (Ag) and gold (Au) are extracted from their ores by forming soluble cyanide complexes (e.g., Na[Ag(CN)2​]), which are then reduced to obtain the pure metal. Also, electroplating (e.g., gold, silver, chromium plating) often uses complex ions for smooth and uniform deposition.

Leave a Reply